Chromated copper arsenate is generally made by dissolving the oxides of the constituent elements, but it could be made, for example, by dissolving copper arsenate in chromic acid. Copper arsenate can be made simply by reacting solutions of a soluble arsenate and copper sulfate that are obtained from metallurgical processes. The arsenate solution may be obtained from the leaching of arsenic-containing materials such as arsenical ores, concentrates, speiss, fumes, flue dusts and the like in an acidic or alkaline medium under pressure and at elevated temperatures, usually in the presence of oxygen, or from other sources such as obtained from solvent extraction of copper refinery electrolyte. Copper sulfate solution may be prepared by dissolving copper sulfate crystals or by leaching copper-containing materials such as concentrates, dusts, slimes, copper scrap, mattes and slags in sulfuric acid.
For example, according to CA Patent 1 094 973, cement copper cake obtained in the zinc dust purification of zinc sulfate electrolyte is leached with acid, the leach residue is leached with caustic at 95.degree. C. while adding air to extract arsenic in solution, and the solution is reacted with copper sulfate to form copper arsenate. CA Patent 1 029 298 discloses a method for making copper arsenate by reacting copper with arsenic acid in the presence of an oxidizing agent and a catalyst. It is mentioned that Ni, Zn, Al and other trace elements are not harmful, but that iron in excess of 1% should be avoided. According to Ger. Offen. DE 3 048 404, smelter flue dust containing Cu, As, Sb, Bi and S is slurried with water and copper sulfate crystals, and the mixture is autoclaved for 3 hours at 120.degree. C. under an oxygen partial pressure of 2 kg/cm.sup.2. Copper arsenate is subsequently precipitated in an alkaline medium. It is disclosed in U.S. Pat. No. 4 405 569 that complex arsenic ores, residues or speisses can be subjected to a two-stage oxidative caustic leach at 150.degree. C., giving solution from which sodium arsenate may be crystallized and converted to copper arsenate, or from which copper arsenate may be directly precipitated.
As pointed out, however, the arsenate and copper sulfate solutions prepared from metallurgical sources, products and intermediates contain sludge-forming impurities, such as lead and iron. Tentative new specifications for copper arsenate require the total lead and the iron contents to be not more than 0.1% by weight. Thus, to produce a relatively pure copper arsenate that is substantially free of sludge-forming impurities, a method must be devised to eliminate these impurities.